Purely from a cost standpoint and all else being equal, singlespeed is much cheaper than geared over the long term. You can get away with replacing chains less often, and you can often find SS chains for cheaper if you're so inclined. But there are cheap ways to go either way as far as initial setup cost goes, and maintenance on a low-to-mid-range geared drivetrain isn't prohibitively expensive.

If you have (or can find for cheap) a rear wheel that's already got threading for a freewheel (instead of a splined freehub body) and you have horizontal dropouts, all you need to make it a singlespeed is a freewheel and a chain, both of which can be very cheap. If you only have a wheel with a freehub, you can still get a spacer kit to make it a singlespeed, but you might spend a bit more. But cassettes can be pretty cheap too, and so can chains.

The real question is, what condition are the derailleurs in, and the derailleur hanger? If the person took them, swapped them, or bent/broke/busted them, the cost to rebuild it with gears goes up pretty quickly.

But yeah, just build it the way you want it. SS is probably cheaper, but probably not by enough to outweigh your actual preference.